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How many videos, outside fixed webcams, come from devices plugged into the wall? I venture that 99.99% of videos where the timestamp is questionable come from battery-powered devices. As for mains hum, portable devices pick up so many hums from so many sources that i doubt much useful mains hum is ever recorded.

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Mains hum is by far the most powerful source of electromagnetic radiation in most indoor places. And the device doesn't have to be connected to the mains to pick up EMI from it - turn up the gain on any battery-powered preamp and you'll hear it.

If you follow the "How it works" link:

> Digital recordings almost always have mains hum on them, either because the device was plugged in to the mains or because it inducts it off nearby cables, lights and appliances in a room.

This is why guitars often use a humbucker pickup to cancel the EMI from the mains.

https://en.wikipedia.org/wiki/Humbucker

You don't pick up the hum from being plugged into the mains. Nearby equipment generates sound at the frequency (and harmonics) which are picked up by the microphone. Various other equipment is sensitive to the EMF. As others mentioned its pervasive indoors and near power lines.

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